EF Core：如何将项目添加到具有一对多关系的导航属性中？

Question

我正在尝试将内容添加到导航属性中。

我想建立一个允许用户将记录输入数据库的系统。一个记录至少有一个或多个许可证。因此，它具有一对多的关系。

我试图通过许可证输入一项工作性质，但出现此错误：

你调用的对象是空的

如何将 Permit 类中的项目传递给 Record 类？

课程

[Table("record" , Schema = "public")]
public class Record
{
    [Key]
    public string Serial_Num {get; set;}
    public string job_nature {get; set;}
    //navigation property
    public ICollection<Permit> permits {get; set;}
}

[Table("permit" , Schema = "public")]
public class Permit
{
    [Key]
    public int id {get; set;}
    public string Serial_Num {get; set;}
    public string permit {get; set;}
    //navigation property
    [Required]
    public Record Record {get; set;}
}

模型构建器

modelBuilder.Entity<Record>()
                .HasMany(p => p.permits)
                .WithOne(r => r.Record)
                .HasForeignKey(p => p.Serial_Num)
                .OnDelete(DeleteBehavior.Cascade);

部分代码（1对1关系试用）

// Record Class
Record objRecord = new Record();
objRecord.Serial_Num = "001";
objRecord.job_nature = "test";

// Permit Class
Permit objPermit = new Permit();
objPermit.Serial_Num = objRecord.Serial_Num;
objPermit.permit = "permit_test";

// I'm trying to include Permit into Record, but returns 'Object reference not set to an instance of an object' error.
objRecord.permits.Add(objPermit);
context.record.Add(objRecord);
context.SaveChanges();

Answer 1

[Table("record" , Schema = "public")]
public class Record
{
    // Add the constructor to initialize permits
    public Record()
    {
        permits = new HashSet<Permit>();
    }
    
    [Key]
    public string Serial_Num {get; set;}
    public string job_nature {get; set;}
    //navigation property
    public ICollection<Permit> permits {get; set;}
}

Answer 2

这个怎么样：

objRecord.permits = new List<Permit>()

然后添加到列表中。